1. Field of the Invention
The present invention relates to a spectrally sensitized silver halide photographic emulsion and, more particularly, to a silver halide photographic emulsion supersensitized by a combination of two types of sensitizing dye.
2. Description of the Prior Art
It is well known in the production of photographic light-sensitive materials to add a certain kind of cyanine dye to a silver halide photographic emulsion to thereby extend the light-sensitive wave-length region to the longer wave-length region, i.e., spectral sensitization is well known.
It is also well known that spectral sensitivity is generally influenced by the chemical structure of the sensitizing dye, various properties of the emulsion such as the halogen composition, crystal habit and crystal system of the silver halide, silver ion concentration, hydrogen ion concentration and the like. Furthermore, the spectral sensitivity is also influenced by photographic additives present in the emulsion, such as a stabilizer, anti-fogging agent, coating aid, precipitating agent, color coupler and the like.
In general, only one sensitizing dye is used in a light-sensitive material to sensitize to a desired spectral wave-length region. When sensitizing dyes are used in combination, there results in many cases sensitivity lower than the spectral sensitivity obtained by the use of a single dye. However, in special cases, spectral sensitivity can be raised in a super-additive manner by using one or more sensitizing dyes in combination with a certain sensitizing dye. This phenomenon is known as supersensitization. However, sensitizing dye groups used in combination are specific to each other, and only a slight difference in chemical structure exerts a great influence on this supersensitizing action. Therefore, useful combinations of sensitizing dyes are difficult to predict merely from the chemical structure.
The sensitizing action produced by a certain kind of sensitizing dye is influenced by the type of emulsion to which the dye is to be added, in particular, the halogen composition, crystal habit and crystal system of the silver halide.
Furthermore, the sensitizing action on a definite emulsion can be varied by changing the conditions in the emulsion. For example, the sensitizing action can be enhanced by increasing the silver ion concentration or decreasing the hydrogen ion concentration, or by varying both factors. Therefore, the sensitizing action can be enhanced by immersing a film having coated thereon a spectrally sensitized emulsion in water or an aqueous ammonia solution. The above-described method for changing the sensitivity of a sensitized emulsion by an increase in the silver ion concentration or a decrease in the hydrogen ion concentration, or by both, is usually referred to as hypersensitization. In general, a hypersensitized emulsion has a short storage life.
In applying supersensitization to a silver halide photographic emulsion, it is required that the sensitizing dyes used exert no detrimental interaction with other photographic additives and have photographic characteristics which are stable during the storage of light-sensitive materials.
Furthermore, it is required the sensitizing dyes used leave no residual color derived from the sensitizing dyes in the processed light-sensitive material. Particularly, it is required that in rapid processing (usually conducted in several seconds to several 10 seconds) that no residual dye be present.
On the other hand, as panchromatically spectrosensitized photographic light-sensitive materials, there are known high speed negative light-sensitive materials, light-sensitive materials for use in copying, and the like. In a light-sensitive material for use in copying, such as microfilms, the colors used in the original range over almost the entire visible spectrum, and, as an exposure light source, one containing light of longer wavelengths in a larger amount, such as tungsten light, is often employed in addition to sun light. For this reason, the intrinsic sensitivity of silver halide is insufficient, and it is necessary to panchromatically spectrosensitize the material. In particular, it is important in microfilms to provide a strong antihalation layer so as to obtain a high resolving power, and hence a panchromatically spectrosensitized silver halide photographic emulsion having high light-sensitivity is required.